Is it permissible to take a loan from a bank in a foreign country and then pay back only the principal amount without the agreed-upon interest, under the pretext of avoiding usury (riba)? And is this considered theft, given that the interest was agreed upon, even if the other party is not Muslim?

The usurious loan contract is forbidden by Islamic law, and it is not permissible to enter into it, even if the intention is not to pay interest. Signing it is a commitment to pay usury, and the Prophet, peace and blessings be upon him, cursed the one who consumes usury, the one who feeds it, its scribe, and its two witnesses. Usury is forbidden and is a major sin, whether it is between Muslims or between a Muslim and a disbeliever, due to the generality of the Islamic legal evidence.